Data mapping for application/www-x-form-urlencoded

[shafreenAnfar]

At the moment we do the data mapping as in #976. But we have missed the below data mapping.

Type	Media Type
map<string>	application/www-x-form-urlencoded

[shafreenAnfar]

I think this a blocker for the release because we have missed a common content-type.

[chamil321]

@ayeshLK Let's add the map type support for the service binding first. Then gradually introduce it to the client. When introducing map<string> if the content type of the annotation is @http:Payload{mediaType : "application/www-x-form-urlencoded"} , get the payload as string and create map. Otherwise, get the payload as json

[shafreenAnfar]

Here is the proposal #2530